For good or bad, his family line (Claudius, Caligula, Nero, Agrippina the Minor), will always be remembered.
As for mediocrity, I think any man who refuses the purple and honors Augustus' wishes is a great man, at least morally. How many generals plundered and cast Rome into Civil War for this very reason?
As for his battles, they all seem to have gone well. His objectives were met, Arminius defanged, and the lost standards recovered. There were some losses, but that is war. Tiberius ended the Campaigns in Germania, most likely out of jealousy or fear of his adopted son's rising popularity and fame. So he did not leave in defeat.
As for Piso, I think Germanicus was just too fair and eager to follow the rules, for his own good. With men like Piso, you cant reason.
I only wish that later soldiers and men of Rome were so mediocre. Makes one wonder what would have happened had he leaved? Would Rome have been subjected to the horrors of Caligula and Nero?
But thats just my opinion. Fascinating thread by the way.
Inqsoc. Would Germanicus have been in control of Syria, or Piso as Governor? I am not certain. I would love to see what resources each had at their disposal.